It is known that an internal combustion using gasoline as fuel reduces the amount of nitrogen oxide (NOx) in an exhaust gas when hydrogen gas is supplied in addition to gasoline. A technology disclosed, for instance, by Japanese Patent Laid-open No. 2004-116398 determines the hydrogen addition ratio so as to reduce the NOx exhaust amount, and operates an internal combustion engine by injecting gasoline and hydrogen in accordance with the determined ratio.
Patent Document 1
Japanese Patent Laid-open No. 2004-116398
Patent Document 2
Japanese Patent Laid-open No. Hei 6-200805
However, the aforementioned conventional technology does not consider the property of gasoline. Therefore, if the gasoline property changes, the combustion within a cylinder may deteriorate. The degree of gasoline atomization within a cylinder decreases particularly when the gasoline is heavy at cold startup. Consequently, cold hesitation is likely to occur, and driveability deteriorates due, for instance, to improper acceleration or engine stop. Further, emission deteriorates if the combustion state deteriorates because of the gasoline property.